This invention relates in general to spool valve control and, in particular, to a self-adaptation scheme for spool valves in an electrohydraulic brake system. Spool valves can be used in electrohydraulic brake systems to control the pressure of brake fluid applied to vehicle wheel brakes.
Traditionally, proportional spool valves are operated in a manner proportional to the voltage applied to the valve's controlling solenoid. The spools within a spool valve moves against a spring force to a position proportional to the voltage applied. As the spool moves within a bore in the spool valve body, lands on the spool (typically) allow or prevent fluid communication between different ports formed in the spool valve body, thereby permitting or preventing flow through various fluid conduits connected to the ports. The operation of, for example, a three-way spool valve is well known in the art, and need not be further discussed here. However, as it relates to this invention, the spool valves either allow or prevent hydraulic fluid flow in a hydraulic circuit, for example, in a hydraulic circuit of a vehicle brake system.
An electrohydraulic braking (EHB) system utilizes electronically controlled valves, pumps, or a combination thereof to augment, replace, or control the base braking operation of a vehicle brake system. Base braking, sometimes referred to as foundation braking, is the basic braking called for by the operator of a vehicle. In base braking, the brake pedal operates a master cylinder, causing the master cylinder to send pressurized hydraulic brake fluid to the wheel brakes of a vehicle. Advanced braking systems, such as EHB systems, have been used to improve the performance of vehicle braking systems by augmenting or replacing the base braking function with other braking operations.
One of the first of many advanced braking functions that has been developed for vehicles was an Antilock Braking System (ABS), which typically involves the operation of valves and pumps to selectively release and re-apply brakes during a braking operation. While typical base braking is commanded by the operator, ABS braking controls the vehicle brakes to recover from and limit skidding of a vehicle's wheels due to braking the wheels harder than permitted by the available coefficient of friction of the road surface. Since pumps and valves are electronically controlled to augment the base braking operation, a vehicle equipped with ABS may generally be said to have an EHB system.
Another advanced braking function that may be accomplished by a properly configured EHB system is VSC (Vehicle Stability Control), which is a system for selectively actuating vehicle brakes to improve the stability of a vehicle during vehicle maneuvers. Other braking applications producing a pressure command input to the present invention include DRP (Dynamic Rear Proportioning—a system for controlling the front to rear proportioning of a vehicle braking command), TC (Traction Control—which typically involves selective application of brakes during vehicle acceleration to recover from and limit skidding of a vehicle's wheels due to accelerating the wheels faster than permitted by the available coefficient of friction of the road surface), ACC (Autonomous Cruise Control—a cruise control system that can actuate vehicle brakes to maintain proper vehicle spacing relative to a vehicle in front) and similar functions.
A subset of electrohydraulic braking systems is electronic brake management (EBM). EHB systems can allow braking to be primarily controlled by the vehicle driver with a conventional master cylinder system. Additionally, an electronically controlled portion of the system operates the brakes under certain conditions, i.e. anti-lock, traction control, etc. In Electronic Brake Management systems, primary braking is controlled electronically. In an EBM system, the vehicle driver or a safety system generates an electronic signal, which in turn operates the pumps and valves to achieve a braking pressure within the system. A pedal simulator creates the effect for the driver of applying braking pressure to the brakes, while actually brake pressure at the brakes is actually created by pumps under electronic control. Such systems typically also provide a back-up braking system in case of a failure of the primary system. Frequently, the back-up system includes a master cylinder manually operated by the driver to provide the hydraulic pressure that actuates the brakes during the failure event.
Regardless of the type of electrohydraulic braking system that is used, a system with proportional spool valves has a control process that controls whether the spool valves are closed, opened to particular flow paths, or intermediately positioned to permit throttled flow through particular flow paths. In order for a control system to properly control the spool valves, it must be configured to account for the forces acting on the valves, the natural characteristics of the valves and be able respond to changes in the valve during braking operations.
As discussed above, proportional spool valves used in the above-described systems typically comprise a valve armature in the form of a spool having lands and grooves defined thereon, an outer sleeve within which the spool moves, with various ports formed in the outer sleeve that are closed off by the lands of the spool or connected together by the grooves of the spool during relative movement of the spool. Proportional spool valves also typically comprise a spring which urges the spool toward a rest position. If an electric current controls the valve, there generally is a solenoid that generates a magnetic field causing the valve armature to be moved from the rest position relative to the outer sleeve. Because a valve can be normally open or normally closed, there are different forces acting upon the valve in its default position. Generally, such forces can be a magnetic force, a spring force, an inlet pressure force, or an outlet pressure force. The inlet and outlet pressure forces will ordinarily vary depending upon the load demanded within the system. In that voltage is proportional to current, it is understood that the use of current controls are to be within the scope of the claims of the present invention.
To balance the forces that are naturally occurring on the valve, so that a particular valve is just opened to either a high pressure source or a reservoir, the closing boundaries must be determined. A closing boundary also compensates for deadband in the system.
Deadband compensation is compensation for spool valve overlap—the distance that the spool must move between the point at which the apply port is just closed and the point at which the release port is just closed, and further movement in the same direction will begin to open the release port. This deadband compensation is used to reduce delays in valve response to an applied voltage when the valve is in its normal position. In proportional spool valve pressure controls, a closing boundary is defined as the minimal (for normally open ports) or maximal (for normally closed ports) voltage required to move the spool to a position in which a land of the spool blocks fluid flow through a port of the sleeve. Closing boundary data, as used for deadband compensation, gives minimal ramp lags but is dependent on how accurately the closing boundary is set. Several factors make it difficult to accurately set the closing boundary under all operating conditions. First, it is difficult to accurately measure the closing boundary. Second, the actual closing boundary changes in a time-variant manner due to operating pressure, temperature and potentially other naturally occurring phenomena. Lastly, the actual boundary varies from one valve to another because of manufacturing tolerances.
In a three-way spool valve, there exist an apply closing boundary and a release closing boundary, both of which are needed in the pressure control to combat hysterisis in the hardware.
To exhibit ideal performance in a system, each valve would have to be trimmed individually to match the closing boundaries. This individualized tailoring process is time-consuming and expensive to conduct for a mass-produced system. Therefore, it is important to devise a boundary self-adaptation scheme to produce the system in a cost-effective manner.
U.S. Pat. No. 6,086,167 to Heckmann, et al. describes a method and device for regulating wheel brake pressure. Pressure is regulated by a regulator generating a driving signal quantity for a pressure-influencing valve arrangement on the basis of the active operating point of the valve arrangement. Given a pressure differential across the valve arrangement, the operating point can be determined from a predetermined current-pressure characteristic curve. The characteristic curve essentially defines a point (at or near zero flow) from which up or down hydraulic flow is utilized to regulate wheel brake pressure.
U.S. Pat. No. 6,030,055 to Schubert improves upon the quality of the pressure control system described in U.S. Pat. No. 6,086,167 and makes manual determination and adjustment of the characteristic curves unnecessary. Primarily, Schubert's invention is based upon the alternative exemplary embodiment of U.S. Pat. No. 6,086,167, where a regulator based on pressure difference between reference pressure and actual wheel brake pressure outputs a pressure correction quantity to the reference pressure, and the corrected reference pressure in turn is used to find activation current from the current-pressure characteristic curve. Schubert describes a process that automatically equalizes the correlation between the pressure difference at a valve and the activation current. The correction quantity occurring in the course of a regulation operation is held within defined limits by appropriate adaptation of the characteristic curves. The limits of the correction quantity are determined as a function of the actual wheel brake pressure and the dynamic ratio of the reference pressure. The characteristic curve for the apply valve is modified during pressure buildup and the characteristic curve for the release valve is modified during pressure reduction. It should be noted that both of the afore-mentioned patents deal with poppet valves.
An estimation approach that estimates boundary deviation should disregard performance changes due to other factors. A system that does disregard such other factors would be beneficial in achieving consistent and convergent estimation. Therefore, an estimation approach based on a different philosophy than that of the patents listed above would provide a more accurate response to a pressure command signal. Consistent estimation would in turn help generate consistent pressure control performance for different types of pressure commands.
One such approach is disclosed in my U.S. Pat. No. 6,692,088, which approach entailed adapting a closing boundary for a proportional poppet valve. More specifically, the approach entailed using the sign of the pressure command derivative over time to determine which one of the two estimators, each associated with a respective one of an apply poppet valve and a release poppet valve for a wheel brake in a vehicle hydraulic brake system (or other hydraulic system) should be updated. Next a modified pressure error is calculated in such a way that steady state pressure error, resulting from feedforward term mismatch, control deadzone, and other factors, is subtracted from measured pressure error. Finally, the modified pressure error is used as the input to the estimators and the boundary table is updated using the resultant boundary deviation estimates. However this approach is not obviously applicable to use in spool valves, due to differences in construction between poppet valves and spool valves.